This is Thomas Mayer's blog about vacuum tube audio, to share updates about new amplifiers and preamplifiers and ELROG vacuum tubes.

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Sunday, December 4, 2011

LCR Phonostage With Variable EQ Curves, Part 3: EQ networks

Hi!

Here is an update on the progress of the mono phonostage with variable EQ. This is a photo of the inside of the finished preamp:

The inside is quite packed and it was a bit tricky to fit everything into the given space. Especially the usage of high quality paper in oil caps made this a challenge.

The photos on the left show the inductances for the EQs. They come with many taps for adjustment of the inductance in small steps. Before mounting, the resistors parallel to the inductance are soldered directly to the coils, between the taps which give the closest inductance to the calculated inductance. The upper photo shows the coils for the low frequency EQs. They get a third resistor soldered to the midpont of the two resistors which are in parallel to the coil. The capacitor to ground is later hooked up to this. The second photo pictures the high frequency coils. Since the high frequency networks only have one time constant, there is no resistor in series to the capacitor to ground. This cap will later be directly connected to the midpoint of the two resistors.

Here is a photo of the phonostage before addition of the EQ networks:

The switches for EQ selection are on the right side. The next photo shows how the coils are mounted above them:

And finally a photo with everything in place and wired up:

Eveything ready for testing and insertion into the wooden enclosure. Stay tuned for part 4 with photos of the completed mono phonostage.